When I looked at the calibreapp.com website, there was an option to test the program for free. It did require entering a company name, your work email, your name and password but from the advertising, there did not seem to be a download associated with the application.
The other alternative is that someone downloaded the calibre-x.yy.zz.dmg file, extracted the calibre.app directory from that and then zipped it up. After all, installing from a .zip file is going to be much easier than installing from a .dmg file.
